This invention relates to a rotary actuator for positioning one or more magnetic heads in a disk drive assembly and more particularly to a moving magnet rotary actuator.
Magnetic disk drive units, used for example in computer systems, generally employ actuators which are either of the rotary or linear moving type. Rotary actuators move the head or heads of a read/write assembly in an arc to position a head at a desired track on a disk. Linear actuators, of course, move the heads in a straight line over the tracks of the disk.
Rotary actuators are generally comprised of a pivotable shaft, to which an arm or series of arms is connected to pivot in unison with the shaft. The arm or arms extend radially outward from the shaft to support the read/write heads at the free end thereof opposite the shaft. In one prior art embodiment, a coil is mounted either on the shaft or on the arm assembly (usually, at an end of the arm remote from the heads); and a permanent magnet is fixedly positioned near and usually located above and below each coil on the actuator assembly. Suitable drive and positioning circuitry is coupled to the coil to supply a drive current thereto. As the coil is energized, magnetomotive forces are generated between the coil and the magnet to drive the actuator and thereby position the heads at the desired track on the disk.
In another embodiment of the prior art, the coil comprises the armature of a squirrel cage motor arrangement, and is located at the base of the shaft. Permanent magnets surrounding the armature act therewith to drive the actuator assembly in an arcuate path.
In still other prior art arrangements, the read/write heads are mounted on support arms, as indicated above, and the arms are driven by a coil mounted to the arm assembly. In such an arrangement, fixed permanent magnets are located above and below the coil; and energization of the coil moves the arm and, thus, the heads.
Disadvantages are encountered in the prior art devices discussed above. Typically, those arrangements in which the coil is mounted on the arm of the head support assembly remote from the read/write heads are bulky and difficult to assemble and service. Furthermore, unwanted torque may be exerted on the shaft due to difficulty in properly balancing the head support assembly.
Moving coil actuators typically are referred to as voice coil actuators. In order to operate these actuators efficiently and with high speed, a large torque must be generated to overcome inertial forces acting on the head support assembly. To generate this large torque, large voice coils have been employed. As the size of the coil increases, proper balancing of the mechanical forces exerted on the shaft is more difficult to achieve. This, in turn, leads to increased wear on the shaft bearings. Wear on the bearings will affect both the accuracy and sensitivity of the track positioning ability of the device.
Disk drive assemblies normally are disposed in closed environments, i.e. sealed enclosures to eliminate dust and other particles from contaminating the disk surface. As a result, the useful life of the assembly has been prolonged. Less wear on the shaft bearings will, of course, reduce the possibility of shortening that useful life. In addition, size is an important factor, as smaller components inside the enclosure will accommodate additional disks, thereby increasing the storage capacity of the system. However, as stated, in order to effectively drive rotary coil actuators, the voice coil must be relatively large and this results in greater power requirements and the generation of more heat. The heating problem is addressed by heat conductive materials in the support, chassis and housing of the disk drive assembly. Also, moving coils exhibit less than favorable mechanical rigidity. Consequently, they are susceptible to vibration and instability during track seek as well as read/write operations. The moving coil actuator is also difficult to assemble, requiring precision in aligning the coil and the magnets so that the movement of the coil between the "sandwiching" magnets is uniform and unimpeded.
Another type of rotary actuator that has been proposed heretofore for magnetic disk drive units of the so-called Winchester type contemplates a stationary coil that is fixedly supported opposite a magnet mounted on the arm assembly. The latter is formed as a rotary body having one or more head-support arms extending outwardly from one end thereof with a magnet mount secured to an opposite end of the body. The magnet mount is formed of iron or other suitable magnetic material; and the coil is supported by a suitable structure of magnetic material. Opposite ends of the magnet are oppositely polarized (N) and (S), respectively, such that flux passes from, for example, the north pole, through a portion of the stationary coil, to the flux conductive coil support, along that support and through the coil to return to the south pole of the magnet. Since the coil support is fixed but the magnet moves, the flux conducting path may be thought of as being fixed because it is substantially defined by the fixed support.
One disadvantage of using a fixed flux conducting path of the aforementioned type is the occurrence of "side pull" forces on the shaft about which the actuator body rotates. That is, since the magnetic flux crosses the gap between the magnet and the coil, a lateral force is exerted on the magnet which tends to pull the body laterally. No oppositely-directed force is exerted on the magnet and, thus, the side pull force is not balanced.
Another disadvantage of using a fixed flux conducting path is the occurrence of a bias force which acts as a component of the rotary force exerted on the body. This bias force is present when the body is rotatably driven as well as when the body is stationary. Thus, when a particular track has information written onto or read from it, thus requiring the body to remain still, the bias force tends to move the body, and thus the read/write head, resulting in a tracking error. Such bias forces must be balanced.